This invention relates to a process for producing single crystals of gallium nitride that can be used as semiconductor substrates for blue light emitting devices. The process is characterized by performing congruent melting of gallium nitride under high-temperature (≧2,200° C.) and high-pressure (≧6×104 atm.) conditions and then slowly cooling the obtained gallium nitride melt under the stated high-pressure condition.
High-quality gallium nitride crystals are needed in the development of devices emitting blue or shorter-wavelength light. Today's gallium nitride based devices are deposited on sapphire or other dissimilar crystal substrates and the many dislocations that occur due to the mismatches in lattice constants and thermal expansion coefficients present difficulty in enhancing the device capabilities. Under the circumstances, a strong need exists for larger and better-quality gallium nitride single-crystal substrates that permit thin film growth by homoepitaxy. However, no single-crystal gallium nitride has yet been obtained that meets this need.
A comprehensive review of the technologies available in the art of interest has been made in several books, including “Aoiro Hakko Debaisu no Miryoku (The Charm of Blue Light Emitting Devices)”, edited and written by Isamu Akazaki, Kabushiki Kaisha Kogyo Chosakai, pp. 107-185, published on May 1, 1997. However, no disclosure is made of single-crystal gallium nitride that satisfies the aforementioned need.
The standard technique for obtaining large single crystals is by slowly cooling the melt until it recrystallizes as is commonly performed with silicon. However, this approach is not applicable to gallium nitride since it decomposes into nitrogen and gallium at elevated temperature. Therefore, in the previous studies, synthesis has been attempted by other methods including vapor-phase growth and the use of sodium flux but no single-crystal gallium nitride has been successfully grown that has a sufficient size and quality to permit use as substrates.
A Polish group has published a method that yields single-crystal gallium nitride by reacting high-pressure nitrogen gas at about 2×104 atm. with liquid gallium. However, this method permits only a slow growth rate and involves difficulty in growing large single crystals. In order to obtain larger and better-quality single crystals of gallium nitride, the growth of a single crystal by slowly cooling the melt is essential and it was by employing high pressure of at least 6×104 atm. that the present inventors enabled the production of single-crystal gallium nitride involving the slow cooling of the melt.